The present invention relates to enciphered electronic communication, and more particularly to systems for real-time scrambling of television signals.
With the continued advance of technology, it has become a relatively simple matter for anyone sufficiently motivated to eavesdrop on an increasing volume of electronic communications. This flow of information takes many forms including, among others, data exchanges between computers, telephone conversations, and television broadcasts. In the last category are conventional television including direct and satellite broadcasts and special-purpose TV including teleconference and surveillance transmissions.
Since some of these TV signals represent sensitive information, a device for enciphering or scrambling the TV signals to render the transmitted information unintelligible to anyone unauthorized to receive it is desirable. U.S. Pat. No. 4,484,027 discloses a system for scrambling subscription television signals to prevent non-paying access by system subscribers and others. The TV signals are combined with the output of a pseudo random number (PN) generator and the result is transmitted to a receiver equipped with a descrambler which has another PN generator. The two PN generators are synchronized and the TV signal is deciphered through a system of user ID codes and a cipher key. The disclosed scrambling system is particularly directed to using ciphers for which the keys are changed on a regular basis and are effective only for non-delinquent subscribers.
Another scrambling system is disclosed in U.S. Pat. No. 4,091,423 which is directed to enciphering conventional synchronous digital data communication such as that used by a document facsimile transmission system. A low-rate digital data stream formed by scanning a document with a photocell is combined with a randomized data stream. The scrambled signal is transmitted to a receiver which includes means for generating a random data stream synchronized with the transmitter. The transmitted scrambled data is deciphered by de-randomizing the data stream with the result being used to reconstruct the transmitted document. However, the data rate of the facsimile signal is only 2400 bits per second which is significantly less than the 4 megabytes per second used for some TV signals.
Neither of the two systems disclosed in the patents completely randomizes a TV signal so that no vestiges of the transmitted pictures remain in the scrambled output. Abrupt brightness transitions such as switching on a lamp or imaging a high-contrast edge are clearly discernible in the scrambled output, thus compromising the covertness of the communication link.